1. Field of the Invention
This invention relates to a process for the treatment and extraction of organic cork compounds using a dense fluid under pressure, and particularly a super-critical fluid.
2. Description of the Related Art
Cork is a natural impermeable and lightweight material derived from the bark of some types of oak such as cork oak, most of which are found in countries bordering the Mediterranean, in Europe and in North Africa.
Cork is used particularly for making bottle corks used to close receptacles such as bottles containing liquids for food consumption, particularly wine.
Bottle corks have been used to close wine bottles for very many years.
About 15 billion bottles are closed in this manner throughout the world every year.
Cork is a natural product that has properties particularly well suited for conserving wine in bottles. Cork is elastic, resilient, compressible, has a high coefficient of friction, is impermeable to liquids, and has a sufficient permeability to gases to enable the exchanges necessary for the product to mature in bottles.
However, occasionally, some alterations to the smell and/or taste of the conserved liquid such as wine compromise the coherence of the natural pair consisting of the cork and the wine.
These alterations are all referred to under the generic term “corky flavour”.
It has been shown that most of these alterations are completely independent of the cork (stopper) and originate in the wine itself or in its preparation and storage.
However, other alterations are caused by the cork (stopper) and a great deal of work has been done throughout the world to find the cause of the problem and to find a solution.
Thus, it has been found that cork tastes can be globally distributed into “genuine” corky flavours, corky flavours, and finally musty tastes.
A “genuine” corky flavour is a putrid taste making the wine undrinkable. This defect is related to yellow stain, in other words growth of a higher fungus, Armillaria mellea. 
Cork boards affected by yellow stain are usually put aside during the first cork board selection sort.
The frequency of this defect is of the order of 1 in 10 000 to 1 in 100 000.
The corky flavour is due to the fact that cork is not inert towards wine. It provides aromatic compounds in variable proportions that can interact positively or negatively with the wine.
The presence of more or less accentuated undesirable tastes is related to preparation of cork, particularly the storage duration in the yard and boiling conditions.
The frequency at which these tastes occur can be minimised by respecting the Code International des Pratiques Bouchonnières (International Cork-cutter Practices Code).
A “corky flavour” and a “musty taste” are very often confused. Musty tastes are fungus, forest undergrowth and mould type tastes related to the presence of organic molecules, the most frequently mentioned being methyl-isoborneol and its derivatives with a “musty and camphrous” smell, geosmine with an “earthy” smell, methylthio-ethyl-pyrazine with a “musty and sulphurous” smell, alcohols and unsaturated cetones in C8 with a “mushroomy” smell, and particularly chloroanisoles and especially 2,3,4,6-TeCA2,4,6-TCA (trichloroanisole) and 2,3,4,6-TeCA (tetrachloroanisole).
These chloroanisoles, which smell very strongly and for which the perception threshold in water is between 0.03 and 4 ng/l, originate from methylation of slightly volatile chlorophenols that have a very mild smell. This reaction is carried out by a very large number of moulds and is equivalent to a chlorophenol detoxification reaction.
Chlorophenol type precursors may originate from different locations and from health care products, insecticide treatments, atmospheric pollution and cork degradation reactions, for example subsequent to some washing operations by hypochlorite.
Finally, particularly to eliminate “corky flavours” as much as possible, cork is subjected to various treatments and particularly cleaning operations.
Cleaning treatments and operations take place at different stages in the production or maintenance of cork objects, for example such as bottle corks (stoppers).
A treatment consists of eliminating micro-organisms from the cork, responsible for the production of undesirable metabolites during the treatments themselves after finishing.
Thus, before the bottle corks are finished, they are washed with chlorine using lime chloride or hypochlorite followed by washing with oxalic acid using a traditional process, or washing with peroxide (hydrogen peroxide or peracetic acid), or using sulfamic acid, or they are washed with metabisulfite using a solution of SO2.
After the bottle corks have been finished (washed or unwashed), they are treated by the injection of SO2, by ethylene oxide or by gamma radiation.
The most frequently used process for eliminating volatile compounds responsible for tastes is the use of hot or boiling water, known as “boiling”.
Thus, in the F. BORDAS process that was first used in 1904, the parts are placed in a chamber heated to 120° C. for 10 minutes and a vacuum is then created, and then the pressure is restored by allowing steam to penetrate.
The chamber is then heated to 130° C. for 10 minutes.
This old method has never been supported by any precise and evaluated data.
The CHAMPCORK process consists of putting parts in a chamber saturated with steam at 130° C. and a pressure of 180 kPa for 18–20 minutes. Atmospheric pressure is then restored.
All these techniques described above have many disadvantages, including:                low efficiency towards organic compounds causing undesirable tastes;        incomplete efficiency towards some micro-organisms.        
Furthermore, most of the techniques mentioned use chemical products that introduce risks, nuisances and constraints, both for the personnel who are using them and for the environment.
Thus, operators are exposed to the risk of inhaling noxious substances, which makes it necessary to wear a mask, whereas the effluents generated by these treatments contain large quantities of compounds containing sulphur and/or chlorine and must be subjected to a long and expensive purification process before they can be rejected.
The document by MIRANDA, Ana M. et al, “High-pressure extraction of cork with CO2 and 1.4 dioxane”, Process. Technol. Proc. (1996), 12 (High Pressure Chemical Engineering), pages 417–422, discloses a process for the treatment of cork using high pressure mixes (170 bars) of CO2 and dioxane within a temperature range from 160° C. to 180° C. Suberine is extracted in this manner.
The conditions of the process disclosed in this document are completely incompatible with use of cork after treatment, particularly for the manufacture of bottle corks.
Document U.S. Pat. No. 5,364,475 discloses a wood cleaning process, particularly a process for extraction of pentachlorophenol (PCP) type compounds or other non-ionic biocide organic compounds in which the wood is firstly cut into suitable size pieces, and is then subjected to a treatment by a fluid in the super-critical state, for example CO2.
A modifying agent or co-solvent chosen among methanol, ethanol and acetone is preferably added to the super-critical fluid in a proportion of 1 to 10% by weight, which increases the extraction efficiency.
The process according to this document is only applicable to pentachlorophenol (trichloroanisole and tetrachloroanisole are excluded) and can only extract contents compatible with potential changes to some environmentalist standards (Toxicity Characteristic Leaching Profile Level) (of the order of 0.1 ppm).
The outer shell of treated trunks is mentioned in this patent although it does not mention if the outer shell consists of cork or bark, since the outer shell described in this patent could also include wood if the impregnation process caused strong penetration into the PCP material. There is a very precise definition of the word “cork” in French and in English, to mean a type of material and also a variety. Cork oak (Quercus suber L.1) is a very particular species not mentioned in the American patent referred to above.
Furthermore, this patent states that the treated product must be divided into particles with a thickness not exceeding 1 to 5 mm but table 7 demonstrates that the extraction efficiency drops very quickly above a thickness of 0.25 mm.
This grinding and shredding step is extremely penalising and it means that the process cannot be used to treat cork boards or bottle corks under any circumstances.
Similarly, document DE-A-4 223 029 discloses a process for extraction of tar oil from old wood or wood waste, in which the wood is firstly ground to a size of 10 to 40 mm, and then is put into contact with a super-critical extraction fluid such as CO2 or an aliphatic hydrocarbon with 3 to 5 carbon atoms for a sufficiently long time to reduce the concentration of tar oil to a required threshold.
A co-solvent such as ethanol or isopropanol can be added to the super-critical fluid with a content of 2 to 5% by volume of the moisture content in the wood.
Document WO-A-98/16288 applies to a process and an installation for the extraction of inorganic and/or possibly organic polluting compounds by a super-critical fluid such as CO2, starting from a material such as wood.
The extracted polluting organic compounds are particularly polychlorobiphenyls, chlorophenols and polychlorophenols (PCP), lindane, polyaromatic organic compounds, insecticides, fungicides, and other additives that might be found in the wood.
A co-solvent such as water may be added to the super-critical fluid in proportions of 5 to 20% by weight.
None of the processes described above using a dense fluid under pressure is applied to cork, but cork is an extremely specific material for which the properties are very different from the properties of wood, mainly in terms of elasticity and density.
It could be added that the chemical composition of cork is very different from the chemical composition of wood, particularly concerning the composition in suberine, lignin and cellulose.
The chemical composition of cork is usually as follows:
46% suberine (the main constituent of cork). The very high compressibility and elasticity of cork are due to the presence of large quantities of suberine.
25% lignin (structure of the cellular walls).
12% cellulose and other polysaccharides. Cork cellulose is in free form.
6% tannins.
6% ceroids. Ceroids repel water and contribute to the impermeability of cork.
5% ash and other compounds.
Refer to the following documents for further information on this subject:
A. Guillemonat. Progrès récents dans l'étude de la constitution chimique du liège (Recent progress in the study of the chemical composition of cork). Ann. Fac. Sc. De Marseille, 1960, 30, 43–54.
H. Pereira. Chemical composition and variability of cork from Quercus suber L. wood Sci. Technol., 1988, 22 (3), 211–218.
The chemical composition of dry wood is usually as follows, for comparison purposes:
60% cellulose.
25% lignin (structure of cellular walls).
15% other compounds.
Cork is a specific material and is fundamentally different from wood due to this difference in its composition.
The study of prior art described above shows that there is a need that has not been satisfied for a process for the treatment or cleaning of cork in order to eliminate contaminating and polluting organic compounds such as chlorophenols and chloroanisoles responsible particularly for undesirable tastes and smells.
There is still a need for a process for cleaning cork in order to selectively eliminate the said contaminating organic products without also affecting some other organic compounds such as suberine, ceroids, lignin and cellulose that confer desirable or even essential properties to the cork, particularly in view of its use for the manufacture of bottle corks.
Preferably, this process must also simultaneously eliminate or limit the presence and proliferation of micro-organisms such as bacteria and fungi in cork.